I like this.
As for explaining the Hulk getting there? Just adapt the story.
- Black Widow and Banner meet up, have an intimate night or whatever because they're in love
- While he's got his guard down she Hulk suppression gasses him or whatever
- Tony and/or whoever else restrain him and ship him into space/into a portal to :shrug: because he's a threat for the on-coming Civil War
- He crash lands here and ends up in this spot
- "Hey Thor, what's up"
Easy peasy. It also sets up him being all pissy and shit when he comes back. Maybe a World War Hulk story could follow? Probably not.
Since Stark and Widow are working with Thunderbolt Ross in Civil War, this is actually kinda plausible.